MyCloud 4TB - Cant recognize external USB3.0 HDD

Yes one can roll back but it may depend on your version. Most if not all roll back methods are for the older first gen v4.x My Clouds. Unknown if those methods will work on the newer second gen v2.x My Cloud. See the following link:
https://community.wd.com/t/wd-my-cloud-v4-x-and-v2-x-firmware-versions-download-links/148533